Citrate toxicity limits rate of performance and duration of peripheral blood stem cell (PBSC) collections and other large volume leukapheresis (LVL) procedures. Knowledge of the metabolic response to citrate infusions and intravenous calcium (Ca) administration during LVL could increase both donor safety and product yields. We performed detailed clinical and laboratory monitoring in 29 allogeneic donors donating PBSC products for clinical transplant use. Twelve to 25 liters of blood were processed at standard (group A) or high (group B) citrate infusion rates. Citrate-related symptoms were scored as barely noticeable (1), irritating (2), uncomfortable (3), or unbearable (4). Subjects in Group A underwent LVL at standard citrate infusion rates of 1.0, 1.2, 1.4, and 1.6 mg/kg/min (24 total donors, six at each infusion rate). Calcium infusions were administered in group A only for symptom scores greater than or equal to 2. Group B subjects underwent LVL using high citrate infusion rates of 1.6 to 2.5 mg/kg/min, with prophylactic Ca administered at a dose of 0.45-0.6 mg Ca per ml ACD-A (23 total donors, five to six at each infusion rate). Calcium infusions were randomized between equimolar Ca chloride or Ca gluconate. The incidence of grade 1 and 2 citrate-related symptoms in group A increased with increasing citrate infusion rates, as follows: 1/6 and 0/6 donors at 1.0, 3/6 and 1/6 donors at 1.2, 4/6 and 2/6 donors at 1.4, and 5/6 and 2/6 donors at 1.6 mg/kg/min. In contrast, no donor in Group B, the cohort receiving prophylactic Ca infusions, had level 2 symptoms, and only three Group B donors had level 1 symptoms, despite receiving markedly high citrate infusion rates. The mean nadir decrease in ionized Ca was minus 30 percent in group A and minus 11 percent in Group B at comparable citrate infusions rates of 1.6 mg/kg/min. Ionized Ca and Mg concentrations were highly correlated with blood citrate levels, which increased with increasing citrate infusion rates and continued to increase throughout the LVL procedure, despite metabolic clearance and urinary excretion. Marked decreases in ionized Mg (20 to 40 percent) occurred, but were not correlated with symptoms. Post-/pre-apheresis urine Ca and Mg ratios became elevated at citrate infusion rates greater than 1.2 mg/kg/min. Equimolar infusions of Ca chloride and Ca gluconate had similar efficacy in mitigating nadir Ca concentrations and preventing symptoms. These studies demonstrate that citrate levels are the main determinant of ionized Ca and Mg levels during LVL. Citrate not only complexes blood Ca and Mg ions, but significantly increases urinary Ca and Mg excretion. Prophylactic Ca infusions, according to a preset algorithm, prevent citrate toxicity and allow higher blood processing rates. These experiments have allowed larger volumes of blood to be processed in shorter times, thus increasing cell yields for transplantation, and have become the basis for new DTM standard operating policies designed to improve collection efficiency, cell yield, and donor comfort during LVL.